Power steering apparatus

ABSTRACT

A power steering apparatus in which relative rotation between two steering shaft members connected by a resilient coupling member actuates a flap valve mechanism, the action of which controls the hydraulic pressure of a hydraulic motor to boost the power applied by manual steering, said apparatus comprising pistons which exert pressure against flap valves so as to damp the vibration of said valves, and a surge pressure absorbing apparatus in the hydraulic pressure circuit which absorbs the surge pressure generated in said circuit.

United States Patent I [is] 3,657,967

Suzuki [451 Apr. 25, 1972 [541 POWER STEERING APPARATUS [56] References Cited [72] Inventor: Akira Suzuki, Kariya, Japan UNITED STATES -rg [73] Assignee: Toyoda Koki Kabushiki Kaisha trading as 2,141,703 12/1938 B Toyoda Machine Works, L d-, K riy 2,684,692 7/1954 Hunter et a1... Aichi, Prefecture, Japan 3,408,900 11/1968 Tomita ..9l/372 [22] Flled: Sept' 1969 Primary Examiner-Paul E. Masiousky [21] App]. No.: 858,380 Attorney-Holcombe,Wetheril1& Brisebois I [57] ABSTRACT [30] Foreign Application Priority Data A power steering apparatus in which relative rotation between Sept. 17, 1968 Japan ..43/67056 two Steering Shaft members connected by a resilient coupling member actuates a flap valve mechanism, the action of which [52] US. Cl ..9l/372, 91/375, 92/136 ontrols the hydraulic pressure of a hydraulic motor to boost [51] Int. Cl 9/10, F0) the power applied by nanua] steering, said apparatus ompris- [58] Field of Search ..91/375 A, 370, 371, 372, 373; i pistons whi h exert pressure against flap valves so as to 186/792; 60/52 S, 52 PJ damp the vibration of said valves, and a surge pressure absorbing apparatus in the hydraulic pressure circuit which absorbs the surge pressure generated in said circuit.

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ll 2L 25 35 34 36 26 POWER STEERING APPARATUS SUMMARY OF THE INVENTION The present invention relates to power steering apparatus, and more particularly to power steering apparatus comprising a hydraulic motor to boost the power applied by manual steering, said apparatus comprising servo valve means wherein flap valves are angularly moved by manual steering to control the pressure fluid for operating the hydraulic motor.

In power steering apparatus in which the steerable wheels are deflected by controlling the hydraulic pressure in a hydraulic circuit through the action of flap valve mechanism due to relative rotation between steering shaft members under the manual steering torque, the slightest rotational displacement of the flap valves can quickly and precisely deflect the steerable wheels. However, in such power steering apparatus, the flap valve mechanism is so sensitive that when a strong turning force acts on steerable wheels, the flap valves begin to vibrate and in consequence the stability of the valve may be impaired, an unpleasant noise may occur, and the steering operation may become unsatisfactory.

Moreover, when the flap valves vibrate or when the steering wheel is turned with a jerk, the gap between the flap valves and the main valve member will change sharply so that an abnormal surge pressure will develop in the hydraulic pressure circuit, and the steering operation of a driver who feels this surge pressure will be adversely affected.

In the present invention, which is designed to eliminate this conventional drawback, a very simple device attached to the flap valve mechanism can absorb the surge pressure generated in the hydraulic pressure circuit as well as effectively damp the vibration of the flap valves.

The present invention stabilizes the valve characteristics of the flap valve mechanism and enhances the steering performance.

The primary object of the present invention is to provide power steering apparatus comprising means for damping the vibration of flap valves.

Another object of the present invention is to provide power steering apparatus in which any surge pressure generated in the hydraulic pressure circuit is absorbed.

Still another object is to provide power steering apparatus which comprises a surge pressure absorbing device in the hydraulic pressure circuit which absorbs the surge pressure generated by a sudden action of the flap valves.

Yet another object is to provide power steering apparatus in which the vibration damping of flap valves and the absorption of surge pressure is attained by a simple mechanism.

The foregoing and other objects of the present invention, which will become more fully apparent from the following detailed description, may be achieved by means of the representative apparatus set forth in this specification and depicted in the accompanying drawings, in which:

FIG. 1 is a longitudinal section through a power steering apparatus constituting one embodiment of the present invention;

FIG. 2 is a transverse sectional view of the apparatus taken along the line IIII of FIG. 1;

FIG. 3 is a sectional view of the apparatus taken along the line III-III of FIG. 2;

FIG. 4 shows a partial enlargement of FIG. 1;

FIG. 5, FIG. 6, FIG. 7 and FIG. 8 are sectional views of the apparatus taken along the lines V-V, Vl-VI, VII-VII, and VlII--VIII in FIG. 4, respectively;

FIG. 9 is a sectional view on an enlarged scale of the essential part of a power steering apparatus in another embodiment of the present invention;

FIG. 10 is a sectional view of the essential part of a power steering apparatus in still another embodiment of the present invention;

FIG. 11 is a sectional view showing part of the apparatus illustrated in FIG. 10 on an enlarged scale;

FIG. 12 is a sectional view of the essential part of a power steering apparatus in a fourth embodiment of the present invention; and

FIG. 13 is a sectional view of the essential part of a power steering apparatus in a fifth embodiment of the present invention.

In FIGS. 1, 2 and 3, reference numeral 11 indicates the main body of a power steering apparatus having a rotating shaft 12 rotatably supported therein. One end of the rotating shaft 12 is connected via a pitman arm and a connecting rod (not shown) and other connecting means to the steerable wheels of a vehicle.

Two axially spaced partial gears 13 and 14 are mounted on the rotating shaft 12. These gears are concentric with the rotating shaft 12 and their teeth extend in opposite directions.

In the main body of the power steering apparatus is a hydraulic pressure cylinder 15. A piston 16 (FIG. 3) is mounted in said cylinder for sliding movement in a direction at right angles to the axis of the rotating shaft 12. At one end of the piston 16 is a rack 18, the bottom of which is supported by a bearing 17.

The rack 18 meshes with the gear 13 on the rotating shaft 12. As shown in FIG. 3, the end of the cylinder 15 is closed by a cylinder cover 19 and a ring 20.

A servo valve housing 21 is attached to the main body 11 of the power steering apparatus. Reference numeral 22 indicates a steering shaft, one end of which is supported in the housing 21 and the other end of whichis supported in the main body 11. The axis of the steering shaft 22 is parallel to the axis of the piston 16. A nut 23 encircles ball bearings 24 which roll ina spiral groove in the steering shaft 22, and rotation of the shaft 22 shifts the nut member 23 in an axial direction against small frictional resistance. The nut 23 is provided with a rack portion 23a, which meshes with the gear 14 on the rotating shaft 12.

In FIG. 4, the servo valve housing 21 is shown to be provided with a pressure fluid supply port 25 axially spaced from a discharge port 26.. Inside the servo valve housing are the supplydischarge ports 27 and 28 which communicate through conduits 59 and 60 respectively with a left compartment 15a and a right compartment 15b of the hydraulic pressure cylinder 15. A main valve member 29 is rotatably mounted within the housing 21, and one end of the main valve member 29 faces toward a coupling member 30. Two axially spaced annular grooves 31 and 32 are cut into the periphery of the main valve member 29 and the supply-discharge ports 27 and 28 open into the grooves 31 and 32 respectively. At the two ends of the main valve member 29 are the valve chambers 35 and 36 housing first flap valves 33 and second flap valves 34, which are rotatable with respect to the main valve member 29 (see FIGS. 5 and 6). In the chamber 35, a valvemember 37, having radial projections which form the first flap valves 33 is so housed as to be able to swing around the axis of the chamber. In the other chamber 36, one end of a steering shaft 38 having radial projections forming the second flap valves 34, is mounted to swing freely around the axis of the chamber. The steering shaft 38 is aligned with the axis of the steering shaft 22 and is resiliently connected via a double torsion-bar 40 (to be described later) to the steering shaft 22. The steering shaft 38 is supported in a sealed bearing member 39 fastened to one end of the servo valve housing 21, and the projecting right end of the steering shaft 38 is connected via couplings and the like (not shown) to a steering column and a steering wheel to be operated by the driver.

The steering shaft 38 with the projections forming the second flap valves 34, the valve member 37 with the projections forming the first flap valves 33, and the coupling member 30 engaging the steering shaft 22 are connected by pins to the double torsion-bar 40 which is coaxial with the steering shafts 38 and 22, which are resiliently coupled together by the double toision-bar 40. The double torsion-bar 40 comprises resilient portion 40a having a relatively small diameter and a small spring constant, and a resilient portion 4011 having a larger diameter and a larger spring constant than that of the smaller diameter portion 400.

The valve chamber 35 which houses the first flap valves 33, as shown in FIG. 5, communicates with the supply. port 25,

and on opposite sides of the first flap valves 33 in the chamber are the distribution ports 35a and 35b.

When the first flap valves 33 are in a neutral position there is a specified gap between the valve seats 33a on opposite sides of the first flap valves and the distribution ports 35a and 35b.

The valve chamber 36, which houses the second flap valves 34, communicates with the discharge port 26 and has, as shown in FIG. 6, a pair of jet ports 36a and 36b on opposite sides of the second flap valves 34. When the second flap valves 34 are in a neutral position, there is a specified gap between the valve seats 34a provided on opposite sides of second flap valves 34 and the jet ports 36a and 36b. The gap formed between the valve seats 34a on the second flap valves 34 and the jet ports 36a and 36b is made slightly wider than that between the valve seats 33a on opposite sides of the first flap valves 33 and the distribution ports 35a and 35b.

Cylindrical holes 41 extend axially through the main valve member 29, as shown in FIG. 4, and terminate adjacent the inner sides of the first and second flap valves 33 and 34. In the cylindrical holes 41 are the slidably mounted pressure-exerting pistons 42 and 43 and a free piston 44 positioned between the pistons 42 and 43. Springs 45 and 46 are inserted while compressed in the spaces between the pressure-exerting pistons 42 and 43 and the free piston 44.

The spaces between the pressure-exerting pistons 42 and 43 and the free piston 44 communicate via narrow holes 47 and 48 respectively with the annular concave grooves 31 and 32 of the main valve member 29. Thereby, the first flap valves 33 and the second flap valves 34 are subjected by the pistons 42 and 43 to pressures proportional to the pressures generated in the left compartment 15a and in the right compartment 15b of cylinder 15, and unstable vibration of first and second flap valves 33 and 34 can be eliminated by the frictional force between the pressure-exerting pistons 42 and 43 and the first and second flap valves 33 and 34, thus making the valve characteristics more stable.

When a surge pressure develops in the hydraulic pressure circuit, for instance, in conduits 59 and 60, the free piston 44 is displaced in response to the surge pressure, thereby absorbing the surge pressure so that the driver will not feel an unpleasant impact. At the same time, the piston 42 projecting into the valve chamber 35, which communicates with the supply port 25, is restricted against displacement to the right in FIG. 4 by a step in the wall of the cylinder hole 41, whereby the piston 42 is prevented from plugging the narrow hole 47.

In FIGS. to 8, the distribution ports 35a opening into the valve chamber 35 (FIG. 5) communicate through fluid conduits 49 with the annular concave groove 31, while the distribution ports 35b communicate through fluid conduits 50 with the annular concave groove 32. Meanwhile, the jet ports 36a opening into the valve chamber 36 communicate through fluid conduits 51 with the annular concave groove 32, while the jet ports 36b communicate through fluid conduits 52 with the annular concave groove 31.

As indicated in FIG. 1, the supply port 25 communicates through a supply pipe 53 with a pressure fluid supply pump 54, while the discharge port 26 communicates through a discharge pipe 55 with a fluid tank 56. The supply pump 54 is driven by the driving means 57 such as an automobile engine, and the delivery pressure is kept at less than a predetermined value by means of a relief valve 58.

When, in the above arrangement, the supply pump 54 is started to supply a pressure fluid through the supply port 25 into the servo valve housing 21, the pressure fluid admitted into the valve chamber 35 passes through the distribution ports 35a and 35b (shown in FIG. 5), through the fluid conduits 49 and 50, through the annular grooves 31 and 32 to the supply exhaust ports 27 and 28 of the hydraulic pressure cylinder 15. Said fluid also passes through the fluid conduits 51 and 52 to the jet ports 36a and 36b. At the neutral positions of first flap valves 33 and second flap valves 34, both the distribution ports 35a and 35b and both the jet ports 36a and 36b are open, and accordingly the greater part of the fluid supplied passes through the jet ports 36a and 36b and discharge port 26 back to the fluid tank 56 to be reclaimed. Therefore, the pressures in the left compartment 15a and in the right compartment 15b, without rising, are balanced with each other and in consequence the piston 16 does not move.

Now, when the steering wheel is turned clockwise, since the resistance of the ground to turning of the steerable wheels is transferred to the steering shaft 22, the smaller diameter resilient portion 40a of the double torsion-bar 40 is first twisted, while the first flap valves 33, together with the steering shaft 38, are turned clockwise relative to the main valve member 29 in FIG. 5. The distribution ports 35b are thereby closed, while the other distribution ports 35a are opened and in consequence the entry of pressure fluid into the distribution ports 35b is restricted.

When the steering shaft is turned further clockwise, the large diameter resilient portion 40b of the double torsion-bar 40 is twisted, and the second flap valves 34 are rotated clockwise with respect to the first flap valves 33. The jet ports 36b are thereby closed, while the other jet ports 360 are opened and in consequence, the greater part of pressure fluid supplied to the servo valve housing 21 by the supply pump 54 is delivered through the first flap valves 33 to the distribution ports 35a only, by way of the jet ports 36b through the fluid conduits 49, the annular concave groove 31, and the fluid conduits 52.

As stated above, the jet ports 36b are closed by the second flap valves 34 and the flow to the discharge port 26 is restricted. Accordingly, the pressure fluid passes through the annular concave groove 31, the port 27 and the conduit 59 to the left compartment 15a of the hydraulic pressure cylinder 15, thereby urging the piston 16 to the right in FIG. 3. Thus, the fluid within the right hand compartment 15b of the cylinder 15 is discharged into the fluid tank 56 through the conduit 60, the port 28, the annular concave groove 32, the fluid conduits 51 and the wide-open jet ports 36a. In this manner, a slight steering torque applied by the driver to the steering wheel is boosted by the piston 16 to assist it in turning the rotating shaft 12, which turns the steerable wheels in right direction.

On this occasion, the pressure fluid supplied to the distribution ports 35a, passing through the fluid conduits 49 and the annular concave groove 31, is introduced through the narrow hole 47 into the cylinder holes 41 in the main valve member 29, thereby causing the pistons 42 to press against the first flap valves 33 and at the same time causing the free pistons 44 to displace, thereby compressing the springs 46 and urging the pistons 43 against the second flap valves 34. Thus, the frictional force developed between the pistons 42 and 43 and the first and second flap valves 33 and 34 damps the instable vibration of the first and second flap valves 33 and 34 to ensure excellent stability. Therefore the driver does not feel the vibration and accordingly an improved steering performance as well as the elimination of unpleasant noise due to vibration results.

Vibration of the flap valves 33 and 34 or a jerky turning of the steering wheel suddenly changes the gap between the distribution ports 35a and 35b and the valve seats 33a and as a result an abnormal surge pressure develops in the hydraulic pressure circuit. The free pistons 44 move in accordance with this surge pressure and the displacement of the free pistons 44 absorbs the surge pressure. Thus, even if a surge pressure develops in the hydraulic pressure circuit, the steering performance can be maintained without the driver feeling this pressure.

FIG. 9 illustrates another embodiment of the present invention which is essentially no different from the preceding embodiment except that, in the former, the vibrations of the first and second flap valves 33 and 34 and the surge pressure are effectively damped and absorbed by an additional throttle valve installed in the hydraulic pressure circuit.

Constrictions 61 and 62 are respectively provided in conduits 59 and 60 which connect the supply-exhaust ports 27 and 28 to. the right compartment 15a and the left compartment 15b of the hydraulic pressure cylinder 15. Otherwise, the arrangement is entirely the same as shown in FIG. 4, so identical symbols are used to indicate the identical parts and further description is omitted here. The vibrations excited in the first and second flap valves 33 and 34 are mostly attributable to the steerable wheels. For instance, when the steerable wheels of a running car are forcibly deflected the mainvalve member 29 is forced to deflect in response to pressure exerted through the rotating shaft 12, the nut member 23 and the steering shaft 22. As a result the gaps between the distribution ports 35a and 35b, and jet ports 36a and 36b of the main valve member 29 and the valve seats 33a and 34a of the first and second flap valves 33 and 34 change to cause an imbalance in the hydraulic pressure acting on both sides of the flap valves 33 and 34, thereby exciting vibrations in the flap valves 33 and 34. It is therefore desirable to arrange the mechanism in such a way that, even when a strong external deflecting force is exerted on the steerable wheels, the main valve member will not be greatly deflected. For this purpose, the constrictions 61 and 62 are provided in the hydraulic pressure circuit. Thus, the action of the piston 16 mechanically connected to the steerable wheels is made slightly sluggish. This arrangementperrnits the strong deflecting force exertedon the steerable wheels to be absorbed, with the small vibration excited in the flap valves 33 and 34 damped by the frictional force due to the push pistons 42 and 43, and the accompanying small surge pressure being absorbed by displacements of free pistons 44. The vibrations of flap valves 33 and 34 and the surge pressure can thereby be reliably and effectively damped.

FIG. illustrates still another embodiment of the present invention, in which there is no free piston 44 between the pressure-exerting pistons 42 and 43, and there is a surge pressure absorbing device inserted in the conduits 59 and 60.

In the embodiment illustrated in FIG.v 10, a compressed spring 45 is inserted between two pressure-exerting pistons 42 and 43. A narrow hole 63 is cut in the pressure-exerting piston 42 contactingthe first flap. valves 33, and a pressure fluid is admitted through the supply port 25 to the back side of the pressure-exerting pistons 42 and 43. In this manner, the first flap valves 33 and the second flap. valves 34 are respectively subjected to pressure from the push pistons 42 and 43, and because of the frictional force between the first flap valves 33 and the pressure-exerting pistons 42 and between the second flap valves 34 and the pressure-exerting pistons 43, unstable vibrations of the first and second flap valves 33 and 34 can be eliminated to stabilize their valve characteristics. The friction generated between the both flap valves 33 and 34 and push pistons 42 and 43 is proportional to the fluid pressure in the valve chamber 35 and it offers a relatively large resistance to vibration by the flap valves 33 and 34. At the same time, a surge pressure absorbing device is inserted in each of the conduits 59 and 60 which connect the right and left compartments 15b and 15a of the hydraulic cylinder 15 with the ports 27 and 28. As illustrated in FIG. 11, valve holes 64 and 65 are provided midway in the conduits 59 and 60, and in the valve holes 64 and 65 are slidably mounted spools 66, 67, 68 and 69, and between the spools 66 and 67 and the spools 68 and 69 compressed springs 70 and 71 are so positioned that the forces exerted by these springs nonnally keep the spools 66, 67, 68 and 69 in positions at both ends of 65 the valve holes 64 and 65. The spools 66, 67, 68 and 69 are respectively equipped with conduits 72, 73, 74 and 75 provided with constrictions which act as throttles, and these conduits provide communication between the right and left compartments of the hydraulic cylinder 15 and the ports 27 and 28. A space 76 is left between the spools 66 and 67 and a space 77 between the spools 68 and 69 and the spaces 76 and 77 are connected by a conduit 78. Occasionally in the hydraulic pressure circuit an abnormal pressure, i.e., a surge pressure develops, which displaces the spool 67 or 69 to compress the spring 70 or 71, which absorbs this pressure. As the displacement of the spool for absorbing surge pressure is accomplished by means of the fluid leaking; out of the space 76 or 77 through the valve hole 64 or 65, the: speed of displacement is extremely slow, and an effective absorption of surge pressure can be counted upon.

FIG. 12 illustrates a fourth embodiment of the present invention, in which the spaces 76 and 77 formed between spools 66 and 67 and 68 and 69 (not shown) are connected through a discharge conduit 79 to the valve chamber 36 which leads to the discharge port 26. a

In this embodiment, just as hereinbefore described, displacement of the spool caused by a surge pressure absorbs the surge pressure.

FIG. 13 illustrates a fifth embodiment of the present invention, which is basically the same as the embodiment illustrated in FIG. 1 except that a free piston 80 is installed independently, separate from the pressure-exerting pistons 42 and 43.

In this embodiment, a valve box 81, secured to the servo valve housing 21, is provided with a valve chamber 82 having a relatively large diameter and a free piston 80 is mounted in thecharnber 82. Compressed springs 83 and 84 are provided on both sides of the piston 80 and the open end of the valve hole 82 is closed by a plug 85. The valve chamber 82 on the left side of the free piston 80 is connected through the port 27 to the annular concave groove 31 and the conduit 59, while the valve chamber 82'on the right side of the free piston 80 is connected through the port 28 to the annular concave groove 32'and the conduit 60.

In this embodiment, in which the free piston 80 is installed in the valve box 81 which is separated from the servo valve housing 21, the valve chamber 82 can be sufficiently enlarged to receive the free piston 80 having a relatively large diameter and accordingly with a large change in the volume of the valve chamber 82 due to the displacement of the free piston 80, it is possible to effectively absorb thesurge pressure, and the values of the spring constants of compression springs 83 and 84, which act on the free piston 80 can be so selected within a wide range that the best absorption of surge pressure may be assured.

According to the present invention, the main valve member is provided with a plurality of cylindrical holes, each of which has a pressure-exerting piston, and these pressure-exerting pistons are urged against a plurality of flap valves by means of a plurality of springs and the friction between the pressureexerting pistons and flap valves can thereby effectively damp the vibrations of the flap valves. Meanwhile, a pressure proportional to the fluid pressure in the hydraulic cylinder which boosts the steering torque is exerted by the pressure-exerting pistons on the flap valves, so that the vibrations of the flap valves can be adequately damped.

A free piston is installed between two pressure-exerting pistons and a displacement of the free piston can absorb the surge pressure developed in the hydraulic circuit.

Moreover, surge pressure absorbing devices such as throttle valves, spools, and free pistons are provided in the hydraulic circuit so that the surge pressure in the circuit may be absorbed. Thus, the valve characteristics of the flap valve mechanism can be improved, unpleasant noise due to flap valve vibration and the drivers unpleasant feeling due to surge pressure in the hydraulic circuit can be eliminated, thereby stabilizing and enhancing the steering performance.

The present invention is not restricted to the details of the several embodiments disclosed herein, since various modifications can easily be made by a man skilled in the art without thereby departing from the basic concepts defined by the following claims.

What is claimed is:

1. In a power steering apparatus comprising a hydraulic motor for boosting manuallyapplied steering torque, which motor is supplied through a hydraulic circuit including supply and discharge passages leading to said motor,

two rotatably steering shaft members aligned with each other and arranged to permit relative rotation therebetween, one of which shaft members is operably connected to control means in said hydraulic circuit and the other of which shaft members is operably connected to a steering wheel, said control means comprising a main valve member aligned between said steering shaft members and mounted within a valve chamber to divide said chamber into two parts,

first and second flap valves mounted within said two valve chamber parts respectively to distribute pressure fluid to said hydraulic motor selectively through relative rotation therebetween, one of said flap valves being connected to rotate with said one shaft member and the other flap valve being driven by said one shaft member through a resilient connection,

at least two passageways in said main valve member extending between said first and second flap valve members, and a pair of axially spaced pressure-exerting pistons slidably mounted in each of said passageways in contact with said flap valves,

the improvement which comprises means for absorbing surge pressures in said hydraulic circuit including free piston means mounted in parallel with said motor between the supply and discharge passages leading to said motor, and

spring means biassing said pistons against said flap valves to dampen vibrations of said valves and biassing said free piston means in opposite directions.

2. Power steering apparatus as claimed in claim 1 further comprising at least one constriction in one of the supply and discharge passages leading to said hydraulic motor.

3. In a power steering apparatus comprising a hydraulic motor for boosting manually applied steering torque, which motor is supplied through a hydraulic circuit including supply and discharge passages leading to said motor,

two rotatable steering shaft members aligned with each other and arranged to permit relative rotation therebetween, one of which shaft members is operably connected to control means in said hydraulic circuit and the other of which shaft members is operably connected to a steering wheel,

said control means comprising a main valve member aligned between said steering shaft members and so mounted within a valve chamber as to divide said chamber into two parts,

first and second flap valves positioned within said two valve chamber parts respectively and adapted to distribute pressure fluid to said hydraulic motor selectively through relative rotation therebetween, one of said flap valves being connected to rotate with said one shaft member and the other flap valve being driven by said one shaft member through a resilient connection,

at least two passageways in said main valve member extending between said first and second flap valve members, and a pair of axially spaced pressure-exerting pistons slidably mounted in each of said passageways,

the improvement which comprises means for absorbing surge pressures in said hydraulic circuit including a free piston slidably mounted between each pair of pressureexerting pistons, one compression spring inserted between each pressure-exerting piston and the adjacent free piston, and ducts in said main valve member between said pressure-exerting pistons and said free piston connecting the passageways in said valve member to the supply and discharge passages leading to said hydraulic motor.

4. Power steering apparatus as claimed in claim 3 further comprising at least one constriction in one of the supply and discharge passages leading to said hydraulic motor.

5. In a power steering apparatus comprising a hydraulic motor for boosting manually applied steering torque, which motor is sup lied through a hydraulic circult including supply and ischarge passages leading to said motor,

two rotatable steering shaft members aligned with each other and arranged to permit relative rotation therebetween, one of which shaft members is operably connected to control means in said said hydraulic circuit and the other of which shaft members is operably connected to a steering wheel,

said control means comprising a main valve member aligned between said steering shaft members and so mounted within a valve chamber as to divide said chamber into two parts,

first and second flap valves positioned within said two valve chamber parts respectively and adapted to distribute pressure fluid to said hydraulic motor selectively through relative rotation therebetween, one of said flap valves being connected to rotate with said one shaft member and the other flap valve being driven by said one shaft member through a resilient connection,

at least two passageways in said main valve member extending between said first and second flap valve members,

a pair of axially spaced pressure-exerting pistons slidably mounted in each of said passageways with a compression spring therebetween which urges said pistons into vibration damping contact with said flap valves,

a valve housing containing said valve chamber, and means for conducting pressure fluid from one of the parts of said valve chamber into the space formed between said two pressure-exerting pistons,

the improvement which comprises means for absorbing surge pressures in said hydraulic circuit including a valve body externally secured to said valve housing, an auxiliary valve chamber in said valve body, a free piston slidably mounted in said auxiliary valve chamber, and compression springs in said auxiliary valve chamber for urging said free piston in opposite directions, the spaces in the auxiliary valve chamber defined between said free piston and the ends of said auxiliary valve chamber being respectively connected to the supply and discharge passages leading to said hydraulic motor.

6. Power steering apparatus as claimed in claim 5 further comprising at least one constriction in one of the supply and discharge passages leading to said hydraulic motor. 

1. In a power steering apparatus comprising a hydraulic motor for boosting manually applied steering torque, which motor is supplied through a hydraulic circuit including supply and discharge passages leading to said motor, two rotatably steering shaft members aligned with each other and arranged to permit relative rotation therebetween, one of which shaft members is operably connected to control means in said hydraulic circuit and the other of which shaft members is operably connected to a steering wheel, said control means comprising a main valve member aligned between said steering shaft members and mounted within a valve chamber to divide said chamber into two parts, first and second flap valves mounted within said two valve chamber parts respectively to distribute pressure fluid to said hydraulic motor selectively through relative rotation therebetween, one of said flap valves being connected to rotate with said one shaft member and the other flap valve being driven by said one shaft member through a resilient connection, at least two passageways in said main valve member extending between said first and second flap valve members, and a pair of axially spaced pressure-exerting pistons slidably mounted in each of said passageways in contact with said flap valves, the improvement which comprises means for absorbing surge pressures in said hydraulic circuit including free piston means mounted in parallel with said motor between the supply and discharge passages leading to said motor, and spring means biassing said pistons against said flap valves to dampen vibrations of said valves and Biassing said free piston means in opposite directions.
 2. Power steering apparatus as claimed in claim 1 further comprising at least one constriction in one of the supply and discharge passages leading to said hydraulic motor.
 3. In a power steering apparatus comprising a hydraulic motor for boosting manually applied steering torque, which motor is supplied through a hydraulic circuit including supply and discharge passages leading to said motor, two rotatable steering shaft members aligned with each other and arranged to permit relative rotation therebetween, one of which shaft members is operably connected to control means in said hydraulic circuit and the other of which shaft members is operably connected to a steering wheel, said control means comprising a main valve member aligned between said steering shaft members and so mounted within a valve chamber as to divide said chamber into two parts, first and second flap valves positioned within said two valve chamber parts respectively and adapted to distribute pressure fluid to said hydraulic motor selectively through relative rotation therebetween, one of said flap valves being connected to rotate with said one shaft member and the other flap valve being driven by said one shaft member through a resilient connection, at least two passageways in said main valve member extending between said first and second flap valve members, and a pair of axially spaced pressure-exerting pistons slidably mounted in each of said passageways, the improvement which comprises means for absorbing surge pressures in said hydraulic circuit including a free piston slidably mounted between each pair of pressure-exerting pistons, one compression spring inserted between each pressure-exerting piston and the adjacent free piston, and ducts in said main valve member between said pressure-exerting pistons and said free piston connecting the passageways in said valve member to the supply and discharge passages leading to said hydraulic motor.
 4. Power steering apparatus as claimed in claim 3 further comprising at least one constriction in one of the supply and discharge passages leading to said hydraulic motor.
 5. In a power steering apparatus comprising a hydraulic motor for boosting manually applied steering torque, which motor is supplied through a hydraulic circuit including supply and discharge passages leading to said motor, two rotatable steering shaft members aligned with each other and arranged to permit relative rotation therebetween, one of which shaft members is operably connected to control means in said said hydraulic circuit and the other of which shaft members is operably connected to a steering wheel, said control means comprising a main valve member aligned between said steering shaft members and so mounted within a valve chamber as to divide said chamber into two parts, first and second flap valves positioned within said two valve chamber parts respectively and adapted to distribute pressure fluid to said hydraulic motor selectively through relative rotation therebetween, one of said flap valves being connected to rotate with said one shaft member and the other flap valve being driven by said one shaft member through a resilient connection, at least two passageways in said main valve member extending between said first and second flap valve members, a pair of axially spaced pressure-exerting pistons slidably mounted in each of said passageways with a compression spring therebetween which urges said pistons into vibration damping contact with said flap valves, a valve housing containing said valve chamber, and means for conducting pressure fluid from one of the parts of said valve chamber into the space formed between said two pressure-exerting pistons, the improvement which comprises means for absorbing surge pressures in said hydraulic circuit including a valve body externally secured to said valve housing, an auxiliary valve chamber in said valve bOdy, a free piston slidably mounted in said auxiliary valve chamber, and compression springs in said auxiliary valve chamber for urging said free piston in opposite directions, the spaces in the auxiliary valve chamber defined between said free piston and the ends of said auxiliary valve chamber being respectively connected to the supply and discharge passages leading to said hydraulic motor.
 6. Power steering apparatus as claimed in claim 5 further comprising at least one constriction in one of the supply and discharge passages leading to said hydraulic motor. 